The invention of high coercivity, high energy product light rare earth-iron (RE-Fe) based permanent magnets is the subject matter of U.S. Pat. No. 4,496,395 issued Jan. 29, 1985 and U.S. Ser. Nos. 414,936 filed Sept. 3, 1982; 508,266 filed June 24, 1982; and 544,728 filed Oct. 26, 1983, all to Croat and assigned to General Motors Corporation. The preferred magnet compositions are based on the rare earth elements neodymium or praseodymium or both, the transition metal iron or mixtures of iron and cobalt, and boron. The preferred compositions contain a large proportion of a RE.sub.2 TM.sub.14 B phase where TM is one or more transition metal elements including iron.
The preferred method of processing such alloys, as disclosed in the above noted patent and patent applications, involves rapidly solidifying molten alloy to achieve a substantially amorphous to very finely crystalline microstructure that has isotropic, permanently magnetic properties. It is believed that the crystallographic ordering range in the highest energy product alloys is commensurate with optimum single magnetic domain size. Overquenched alloys can be annealed at suitable temperatures to cause grain growth and thereby induce magnetic coercivity. The maximum magnetic energy product to date for an as-quenched Nd-Fe-B based alloy is about 15 megaGaussOersted.
U.S. Ser. No. 520,170 to Lee filed Aug. 4, 1983, now abandoned and also assigned to General Motors Corporation, relates to a method of introducing anisotropic magnetic properties in rapidly solidified RE-Fe-B based alloys by hot-working. Alloys with overquenched, substantially amorphous microstructures are worked at elevated temperatures to cause grain growth and crystallite orientation which result in substantially higher energy products than in the best as-rapidly-solidified alloys. The maximum energy product to date for hot-worked, melt-spun Nd-Fe-B alloy is about 45 mGOe, although energy products as high as 64 mGOe are theoretically possible.
It is also known that high coercivities and energy products can be induced in light RE-Fe-B based alloys by the conventional orient-press-sinter (OPS) method which has been used for more than twenty years to make samarium cobalt and other rare earth cobalt permanent magnets. This method has severe commercial limitations because it entails many processing steps which must be carried out in a controlled, non-oxidizing atmosphere. High coercivities and energy products (greater than 10 mGOe) are obtained only by grinding crystalline alloys into very fine (less than 5 micron) powder, magnetically aligning and compacting the powder in a strong magnetic field, and sintering the compacts. Magnets made by the OPS process are brittle and generally require a substantial amount of finish grinding to achieve any but the simplest small shapes.
It is also known that by substituting copper for some of the cobalt in RECo.sub.5 and RE.sub.2 Co.sub.17 compositions, coercivity can be induced in as-cast alloys by suitable heat treatment between 400 and 500.degree. C. The coercivity is believed to be caused by a domain wall pinning phenomenon. Analogous precipitation hardening of cast RE-Fe-B based compositions has not been achieved to date and the presence of copper in more than trace amounts has been found to drastically reduce the energy product of RE-Fe-B based alloys. It is also well known that certain as-cast Cr-Co-Fe compositions which do not contain rare earth elements can be uniaxially worked at cold temperatures to achieve energy products up to about 5 mGOe.
While the rapid solidification and hot-working of RE-Fe-B based compositions appears to be a more practical and less expensive process than OPS, it would also be desirable to have a means of making high coercivity, high energy product RE-Fe-B based permanent magnets without initial rapid solidification. By rapid solidification herein is meant solidification of an alloy initially at a temperature above its melting point at a cooling rate of about 10.sup.5 .degree. C. per minute or greater. That is, it would be advantageous to make permanent RE-Fe-B magnets from conventionally cast, fully crystalline alloys but without the fine grinding and aligning steps of the OPS process. Conventional casting results in cooling rates substantially lower than 10.sup.5 .degree. C. per minute. However, before this invention, no such method was known or had been suggested.